Numerous temporary image receptors are known in the art of printing. For example, in offset printing intermediate transfer blankets are used to temporarily store a printed liquid toner image prior to transferring that image to a final receptor. Temporary image receptors are also used for electrographic imaging, which is known in the art to include electrophotographic, electrostatic and ionographic printing.
1) Electrophotography:
Electrophotography forms the technical basis for various well known processes, including photocopying and some forms of laser printing. The basic electrophotographic process involves placing a uniform electrostatic charge on a photoconductive element (also referred to as a photoconductor element or a photoreceptor), imagewise exposing the photoconductive element to activating electromagnetic radiation, also referred to herein as "light", thereby dissipating the charge in the exposed areas, developing the resulting electrostatic latent image with a toner, and transferring the toner image from the photoconductor element to a final substrate, such as paper, either by direct transfer or via an intermediate transfer material. Liquid toners are often preferable because they are capable of giving higher resolution images.
In electrophotographic printing, particularly liquid electrophotographic printing, the temporary receptor is a photoreceptor. The structure of a photoreceptive element may be a continuous belt, which is supported and circulated by rollers, or a rotatable drum. All photoreceptors have a photoconductive layer which conducts electric current when exposed to activating electromagnetic radiation. The photoconductive layer is generally affixed to an electroconductive support. The surface of the photoreceptor is either negatively or positively charged such that when activating electromagnetic radiation strikes the photoconductive layer, charge is conducted through the photoconductor in that region to neutralize or reduce the surface potential in the illuminated region.
Other layers, including surface release layers and interlayers, such as priming layers, charge injection blocking layers, barrier layers may also be used in some photoreceptive elements. These photoreceptors are typically multilayer constructions comprised of an underlying photoconductive layer sensitive to actinic radiation and various top coats which impart barrier and/or release properties to the photoreceptor. See R. M. Schaffert, "Electrophotography" (John Wiley: NY, 1975), pp. 260-396.
When multi-colored images are desired, one may apply each toner color to the photoconductor element and transfer each color image to the final substrate separately. Alternatively, all the colors may be first assembled in registration on the photoconductor element and then transferred to a final receptor, either directly or via an intermediate transfer element. This method is referred to herein as simplified color electrophotography (SCE). See e.g. WO97/12288, (incorporated herein by reference). Specifically, a photoreceptor is movably positioned to pass at least one exposure station and at least one developing station. If there is only one exposure station or one developing station, the photoreceptor will have to move past the stations several times to create a multicolor image on the photoreceptor, e.g. two or more rotations. If there are several exposure and developing stations the image may be created in a single pass of the photoreceptor. To begin creating a multi-color image, any previously accumulated charge is erased from the photoreceptor. The photoreceptor is charged to a predetermined charge level. The photoreceptor is first image-wise exposed to radiation modulated in accordance with the image data for one of a plurality of colors in order to partially discharge the photoreceptor to produce an image-wise distribution of charges on the photoreceptor corresponding to the image data for the one of the plurality of colors. A first color liquid toner is applied to the image-wise distribution of charges on the photoreceptor to form a first color image. The photoreceptor may then optionally be recharged by any known means, e.g. by corona charging, or the application of the first toner liquid may itself recharge the photoreceptor to a second predetermined charge level. The exposure, liquid toner application and optional recharging steps are repeated as necessary for each desired color.
A problem that may arise during electrophotographic imaging is poor transfer from the photoreceptor to the intermediate transfer member. Poor transfer may be manifested by images that are light, speckled, fuzzy, or smeared. These transfer problems may be reduced by the use of a surface release coating on the photoreceptor.
The release layer may be applied over the photoconductive layer or over an interlayer. The release layer should be durable and resistant to abrasion. The release layer should also resist chemical attack or excessive swelling by the toner carrier fluid. The release layer should also not significantly interfere with the charge dissipation characteristics of the photoconductor construction. Other desirable attributes of release surfaces include good adhesion to the underlying interlayer or photoconductor, excellent transparency to actinic radiation (i.e. laser scanning devices), and simple manufacturing processes and low cost.
Surface release layers are commonly low surface energy coatings such as silicones, fluorosilicones or fluoropolymers. Various silicone release layers useful as topcoats on photoreceptive elements are described in PCT Patent Publication No. WO96/34318 as well as U.S. Pat. No. 4,600,673, U.S. Pat. No. 5,320,923 and copending U.S. Pat. No. 5,733,698 all of which are incorporated herein by reference.
For liquid electrophotographic printing in particular, it may be desirable to avoid beading of toner excess carrier liquid on the surface of the release layer because the beads of carrier liquid can disturb the toner image. Specifically, the presence of the toner carrier liquid on the surface may allow the toned image to continue to flow with adverse effects on image resolution. Moreover, when a multi-color image is formed on the photoreceptor in a single pass without drying between imaging stages, such beading may cause diffraction of the exposing light during imaging resulting in lack of sharp lines or clarity in the final image. Therefore, release layers which control the liquid on the surface of the photoreceptor are needed. However, the liquid toner should not cause smearing or diffusional broadening (i.e., blooming) of the image. Desirably, the surface release layers permit virtually 100% image transfer from the photoreceptor to an intermediate transfer member, thereby maintaining optimum image quality eliminating or reducing the need to clean the photoreceptor between images.
Color liquid electrophotography, particularly SCE, imposes a number of critical requirements on the release surface of the photoreceptor. The photoreceptor release surface must, in general, provide a low energy surface for transfer of the toner. Moreover, systems that rely on differential adhesive transfer rely on the relationship of the surface energies of the photoreceptor surface, the liquid toner, the toner film, and any rollers that contact the toner surface. See, for example, copending, coassigned U.S. Pat. No. 5,652,282 (Baker et al.) incorporated by reference herein. For some systems, the relative surface energies should be in the following hierarchy from the element with the lowest surface energy to the element with the highest surface energy: drying element, release layer of photoreceptor, intermediate transfer material, toner, final receptor.
Most references related to chemical modifiction of release surfaces for photoreceptors focus on specific combinations of silicones or fluorosilicones coated as thin (&lt;3 micrometers thick) layers from solvent-based formulations. PCT Patent Publication WO 96/34318 discloses a combination of a silicone with a relatively high molecular weight polymer, optionally, a silicone with relatively low functionality, and a crosslinking agent, the ratios of which may be varied in order to modulate or vary release surface properties. These low swelling release surfaces exhibit a bimodal distribution of chain lengths between crosslinks.
Various means are also known in the art for modifying silicone rubbers, for example, by adding particulate fillers to reinforce and thereby increasing the durability and abrasion resistance of the silicone. See Siloxane Polymers, S. J. Carlson and J. A. Semlyen, eds. (PTR Prenticer Hall: NJ, 1993), pp. 512-543 and 637-641. In addition, U.S. Pat. No. 5,212,048 discloses two-component dual cured (addition and condensation cured) silicone coating formulations containing various conductive fillers (e.g. ZnO, Fe.sub.3 O.sub.4 and SnO.sub.2) used to enhance conductivity in non-contact spark discharge imaging of planographic printing plates.
Art related to modification of release surfaces on temporary image receptors by incorporating fillers is described in the U.S. Pat. No. 5,733,698 (Lehman et al), wherein swellable release layer compositions, including compositions based upon high molecular weight hydroxy-terminated siloxanes are generally disclosed. The disclosed release layers are preferably swellable polymeric materials exhibiting swelling behavior in the toner carrier liquid of greater than 40% by weight of the polymer and more preferably greater than 60% by weight.
The same Lehman et al. application also discloses photoreceptor release surfaces in which the surface is roughened to prevent beading of the carrier liquid on the surface. Lehman et al. teach through their examples that the surface roughness (Ra) should be greater than about 10 nm to avoid beading of the carrier liquid. The degree of roughness of the release layer must not be so high as to disturb print quality and should be less than 500 nm, more preferably less than 100 nm, most preferably less than 50 nm. Lehman et al. further disclose that there are various means for obtaining a roughened release surface on a photoreceptive element, including addition of particulates to the release surface. Lehman et al. teach that low surface energy fillers are preferred.
2) Electrostatic Imaging
While the foregoing discussion has focused on the problems associated with surface release layers on photoreceptors in liquid electrophotographic imaging, additional deficiencies with temporary imaging receptors used in other liquid toner imaging processes, particularly liquid electrostatic printing, are known to exist. In electrostatic printing, an electrostatic image is formed by (1) placing a charge onto the surface of a dielectric element (either a temporary image receptor or the final receiving substrate) in selected areas of the element with an electrostatic writing stylus or its equivalent to form a charged image, (2) applying toner to the charged image, (3) drying or fixing the toned image on the dielectric, and optionally (4) transferring the fixed toned image from the temporary image receptor to a permanent receptor. The surface release layer can be transferred with the fixed toned image to the final receptor or can remain on the temporary image receptor after the image transfer to the final receptor. An example of a liquid electrostatic imaging process which makes use of all four steps is described in U.S. Pat. No. 5,262,259. Suitable surface release layers useful in such electrostatic imaging processes are described in European Patent Application 444,870 A2 and U.S. Pat. Nos. 5,045,391 and 5,264,291.
The surface of the dielectric element is typically chosen to be a release layer such as silicone, fluorosilicone or fluorosilicone copolymer. The release layer should be durable and resistant to abrasion. The release layer should also resist chemical attack or excessive swelling by the toner carrier fluid. The release layer should also not significantly interfere with the charge dissipation characteristics of the dielectric construction. It will be understood by those skilled in the art that other properties could be important to durable release performance in liquid electrostatic printing other than those described herein.
One common problem that arises during electrostatic imaging is the phenomenon of carrier liquid beading on the temporary image receptor. Since electrostatic imaging processes typically make use of non-optical means (e.g. an electrostatic stylus or an array of styli) to generate the latent electrostatic image on the surface release layer of the dielectric element, such carrier liquid beading does not generally cause problems of image degradation in multicolor imaging processes due to diffraction of an exposing radiation source as may occur in liquid electrophotographic imaging. However, carrier liquid beading can still degrade image quality by causing the wet toned image to diffusionally broaden or flow, with adverse effects on image resolution. Such image degradation is commonly referred to in the art as "bleeding" of the image.
Another problem which arises in multicolor liquid electrostatic imaging relates to removal of a portion of one color toner layer during the application of a second color toner layer due to contact of the first, still wet toner layer with the electrostatic styli. This phenomenon is commonly referred to in the art as "head scraping."
Yet another problem which arises in multicolor liquid electrostatic printing processes, particularly as described in U.S. Pat. No. 5,262,259, relates to the final transfer step of the fixed toned image from the temporary image receptor to a permanent receptor. This transfer process is commonly carried out using heat and/or pressure. This transfer process is inherently slow, and its speed is limited by the rate at which heat can be transferred through the temporary image receptor and by the upper limit of pressure which can be applied during the transfer step. If the applied heat and/or pressure are not correctly selected, or the transfer speed is too high, poor image transfer can result. Poor image transfer may be manifested by incompletely transferred images or images that are light and/or speckled.
Therefore, there is a need for release layers which control the liquid on the surface of the dielectric receptor and minimize the beading effect. There is also a need for surface release layers which permit virtually 100% image transfer from the temporary image receptor (e.g. dielectric element) to a permanent receptor. There is also a need for surface release layers which permit image transfer from the temporary image receptor to the permanent receptor at higher transfer speeds and at lower temperatures and/or pressures.
3) Additional Information
Art related to chemical modification of release properties is primarily related to the preparation of low adhesion backsides (LAB's) for use in preparing pressure sensitive adhesive tapes or films. Low viscosity addition-cured vinyl silicones are disclosed in U.S. Pat. No. RE. 31,727. The use of ethylenically unsaturated silicone monomers or prepolymers in combination with alkenyl functional silicone gums to obtain low coefficient of friction silicone release are also described in U.S. Pat. No. 5,468,815 and in coassigned European Patent Publication 0 559 575 A1, incorporated by reference herein.